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I built a new cold side for my car move the intercooler from in the front bumper to in the passenger floor, went from 3.5" dia. to 4" dia. and changed the down pipe from a single 5" to a 5" split into two 4" ..................... Now the car is harder to spool up and will not 60ft for anything. I went from 1.20's to 1.40's 60ft and 4.90's & 5.0's at 145-147mph to 5.20-5.40's at 140-144mph
My question is will the 1/2 inch difference in the pipe size make that much difference or is it a combo between the pipe size and the added distance between the turbo to the engine ?
Considering before i had mabe 4 feet of 3.5 tubing in the car and now i have around 7 or 8 feet of the 4 inch. Just looking for any suggestions Thanks
 

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Yes it can. Too large of tubing can kill your midrange power curve. I'm guessing you're making ~1200ish? 3" is probably sufficient for that.
 

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I would put the old downpipe back on or disconnect the downpipe for a transbrake test to see if it helps also! I changed my downpipe last year and killed spool up!

Just something to try
 

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What size is the compressor outlet on your turbo?
 

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There are variables to consider that I wasn't even thinking about last night. On an 800whp car I've seen 20-30hp differences between 2.5" and 3" plumbing while basically having the same peak number. Midrange always suffered with the larger tubing.

One thing I was not thinking of last night that I typically take for granted. These cars have an intake manifold with a decent sized plenum. Some of you guys may not have much plenum volume with the carb intake and elbows that are so popular and that is where you may need the larger plumbing. When the throttle blade is open the charge plumbing essentially acts as a plenum for the engine. If you don't have a sheet metal intake or your intake does not have much plenum volume you can try to make up for it with larger charge plumbing. Going to small will hurt peak numbers and too large will hurt midrange. Have to find that happy medium.
 
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